1. Field of the Invention
The present invention relates generally to an electronic LCD backlighting inverter circuit suitable for LCD backlighting or the like, and more particularly, to an LCD backlighting inverter circuit which is highly efficient, has a low profile, and a wide dimming range.
2. Description of the Related Art
LCD backlighting applications demand efficient, low profile backlighting for information display. Narrow diameter cold-cathode fluorescent lamps (CCFL), such as the T1 type for example, are widely used in the industry for such applications. To drive these CCFLs, high frequency electronic LCD backlighting inverter circuits having high efficiency, low profile, and a wide dimming range are in demand. Presently, voltage-fed half bridge resonant converter circuits, as shown in FIG. 1, and current-fed push-pull resonant converter circuits, as shown in FIG. 2 are used to drive CCFL and other fluorescent lamps. Despite their wide use, these circuits have shortcomings which make them less than optimum solutions for driving CCFL""s and the like. For example, the efficiency of these circuits is optimum and their dimming range is limited. In particular, a disadvantage of the prior art circuit configuration of FIG. 1 is a high output transformer turns ratio, which translates to a higher primary side winding current which leads to higher conduction losses. A further disadvantage of the circuit of FIG. 1 is that the high turns ratio in the secondary winding requires a reduced wire size (e.g., to 44 AWG) which contributes to higher conduction losses in the winding. In addition, a smaller gauge wire may cause problems during manufacturing. Another disadvantage of using a high turns ratio transformer is a significant increase in parasitic capacitance which leads to low efficiency. The typical electrical efficiency of the circuit of FIG. 1 is about 84% (i.e., output power/input power).
FIG. 2 is another prior art circuit configuration of a widely used electronic ballast for driving CCFLs. The backlight inverter of FIG. 2 has a smaller output transformer turns ratio than that described with reference to the circuit of FIG. 1, and is capable of current based lamp power dimming using a Buck regulator stage. While the smaller output transformer turns ratio will lead to smaller losses in the push-pull power stage, the total circuit efficiency is limited by the Buck regulator stage. Another disadvantage of the circuit of FIG. 2 is a narrow dimming range due to the thermometer effect in the LCD panel when the lamp current frequency is high. At higher frequencies, a parallel parasitic capacitance in the lamp shield draws more current from the lamp causing one end of the lamp to be bright and the other to be dim.
In order to improve circuit efficiency and achieve a wide dimming range it has been proposed to use the push-pull resonant inverter stage operated in a low frequency pulse width modulation (PWM) dimming mode and using the push-pull converter switches, Q1 and Q2, in FIG. 2 also as the low frequency switches for PWM dimming. However, the typically high inductance of L1 limits the circuit start up performance and limits the dimming range.
Accordingly, a need exists for an improved electronic LCD backlighting inverter circuit which is more efficient than a conventional electronic LCD backlighting inverter circuits having a wide dimming range, and low profile.
In accordance with the present disclosure, an improved electronic LCD backlighting inverter circuit for use in LCD backlighting applications is provided which obviates the problems associated with the prior art.
According to one aspect of the invention, there is provided an improved high frequency electronic LCD backlighting inverter circuit for powering a fluorescent lamp that is efficient, has a low profile, and a wide dimming range.
The improved high frequency electronic LCD backlighting inverter circuit can operate a load composed of cold cathode flourescent lamps or hot cathode flourescent lamps.
It is a feature of the present invention that the LCD backlighting inverter circuit is optimally designed for high frequency switching, however, the invention provides capabilities for low frequency pulse-width modulated (PWM) switching using logic control circuitry to achieve a wider frequency range than can be realized in conventional LCD backlighting inverter circuits. By controlling the dimming range via logic control circuitry the need for a current driven front end Buck regulator stage, as used in conventional current driven push-pull circuits is removed. That is, the present invention removes the need for a Buck regulator stage switching transistor and associated diode which contribute to low circuit efficiency. Further, the output transformer turns ratio is greatly reduced leading to higher circuit efficiency. Higher circuit efficiency is further realized by selecting an inductance value for L1 that is orders of magnitude lower than the value required in a conventional design. By choosing a smaller inductance value for L1, the inductor does not act as a current source, but instead is considered part of an LLC resonant circuit thereby providing the ability to switch the inventive circuit off at the zero-crossings of the inductance current. By selecting a small value for L1, the problems associated with limitations in circuit startup performance and limitations in achieving a wide dimming range are removed.
An improved electronic LCD backlighting inverter circuit is provided for performing high frequency dimming with a low frequency modulation. The improved electronic LCD backlighting inverter circuit is preferably a voltage-fed push-pull LLC resonant circuit which includes: an LLC resonant circuit including a resonant inductor, a magnetizing inductor and a resonant capacitor; switching means for operating said LCD backlighting inverter circuit at a high frequency modulated by a low frequency signal; low frequency signal generator means for generating a low frequency signal, said low frequency signal having positive and negative going portions; logic means for controlling said switching means and being driven from said low frequency signal, said logic means for extinguishing the operation of said switching means during said negative portion of said low frequency signal thereby causing said electronic LCD backlighting inverter circuit to be frequency modulated by said low frequency signal.